Means for detecting small electric currents



May 15, 1923. 1,455,458

J. 5. E. TOWNSEND MEANS FOR DETECTING SMALL ELECTRIC CURRENTS Filed Oct.15, 1918 um I Patented May 1 5, 1923.

' UNITED sures JOHN SEALY EDWARD TOWNSEND, OF OXFORD, ENGLAND.

MEANS FOR DETECTING SMALL ELECTRIC cuRREN'rs.

Application filed October 15, 1918. Serial No. 258,287.

To all whom it may concern:

Be it known that 1, JOHN SEALY EDWARD TOWNSEND, a subject of the King ofGreat Britain, residing at 20 Merton Street, Oxford, England, haveinvented new and useful Improved Means for Detecting Small ElectricCurrents of which the following is a specification.

The object of this invention is primarily to provide a simplifiedinstrument whereby the wave length of electrical oscillations can bereadily measured. The instrument may, however, be used merely fordetecting the presence of small oscillating currents.

j It has been before proposed to employ an incandescent electric lampbulb for detecting electric currents, but hitherto it has been possibleto use such a detector only when the current is fairly large: that is,large enough to heat the filament from its normal condition to thetemperature at which it becomes incandescent.

According to this invention I employ an evacuated bulb having itsfilament connected in series with a choke coil and a-battery ofsufficient strength to heat the filament to a temperature slightly aboveor below the point of incandes'cence, so that the lamp is in a sensitivecondition toshow visually any small change of temperature due to thepassage through the filament of a small current, and I superimpose thecurrent to be detected upon the current flowing from the battery throughthe filament, so that the addition of this small current to the normalcurrent may cause a change in the condition of the filament, whichchange is easily distinguishedby the eye. Such a detector .is especiallysuitable for showing alternat ing currents or-currents of highfrequency, and may be used either to detect the currents or to indicatewhen they attain a maxi mum value.

My invention is illustrated by the accompanying diagrams.

Figure 1 is a diagram of the connections in one form of my invention andFigures 2 and 3 are diagrams of modifications.

In Figure 1, G is a low resistance lamp having 1ts filament connected inan oscillatory circuit comprising a condenser C and an inductance L. Inseries with the filament are also connected a cell B and a choke coil KThe battery is of such a strength as to maintain the filament of thelamp at a temperature just below that of pacity, and provided with alarge number of turns so that its impedance for high frequency currents.is large as compared with the resistance of the lamp filament. \Vhentherefore an oscillatory current flows through the circuit C(JrL thetemperature of the filament is raised and the filament glows.

Thus the ranged near an aerial A, in which oscillations are supposed tobe occurring. lVhen the aerial is in tune with the circuit GGL thefilament will be at its brightest.

The oscillatory circuit may be tuned by making either the condenser orthe inductance adjustable; when this circuit is in tune with theoscillations to be detected the brightness of the filament is at amaximum, and therefore the arrangement ma be used as a wave meter, andthe wave ength of the oscillations can be determined.

If the circuits are placed in proximity to a closed oscillatory circuitA in which oscillations are flowing, the lam G will be at its brightestwhen the circuits are in tune.

Vhat I claim is 1. In means for detecting the presence of smalloscillating currents, the combination of an evacuated bulb, a filamentcontained therein, a choke coil and a batte both connected in serieswith the filament and an oscillato circuit in which can flow thecurrents to e. detected, the filament being connected in that circuit,the battery being adapted to heat the filament near to the point ofincandescence and the choke coil having large impedance compared withthe resistance of the filament for the whole range of frequencies of theoscillatory circuit.

2. In means for detecting the presence of small oscillating currents,the combination of an evacuated bulb, a filament contained therein, abattery connected in series with the filament and adapted to heat itnear to the point of incandescence, a choke coil connected in serieswith the battery and filament, and an oscillatory circuit in which canflow the currents to be detected, a portion of the coil being includedwith the filament in the circuit, the remainder of the coil having largeimpedance compared with figure shows the circuits arthe resistance ofthe filament for the Whole range of frequencies of the oscillatorycircu-it.

3. In means for detecting the presence of small oscillating currents.the combination of an evacuated bulb, a filament contained therein, abattery adapted to heat the filament near to the point of incandescence,a coil connected in series with the battery and filament, and anoscillatory circuit coupled to the coil.

4. In a Wave meter, the combination of an evacuated bulb, a filamentcontained therein, a cell connected to the filament and possessing avoltage slightly less than that required to heat the filament toordinary incandescence, a choke coil connected in series with the celland :the filament and an oscillatory circuit in which the filament isincluded and in which can flow the oscillations to be measured, thechoke coil having a large impoint of incandescence, and means whereby asmall current to be detected by variations in the brightness of thefilament can be received and superimposed on the battery current throughthe filament.

6. In means for receiving and detecting small electric currents, thecombination of an evacuated bulb. a filament contained therein, abattery connected in series With the filament and'maintaining it nearthe point of incandescence, anda second circuit connected to saidfilament. said secondcir cuit being adapted to receive and have flowtherethrough the small currents to be detected.

7. The method of receiving and detecting small electric currents, whichconsists in superimposing such currents upon a battery current passingthrough a filament contained in an evacuated bulb, maintaining thefilament near the point of incandescence, and observing the existence ofsuch currents by variations in the point of incandescence of thefilament.

In testimony that I claim the foregoing as my invention. 1- have signedmy name this 18th day of September, 1918.

JOHN SEALY EDWARD TOWNSEND.

